Zahra Kouchaki

Characterizing dynamic properties of retinal vessels in the rat eye using high speed imaging.

PURPOSE The dynamic properties of retinal vessels including pulse wave propagation and pulsatility index provide new perspective in retinal hemodynamic analysis. In this study we utilize a high speed imaging system to capture these characteristics in the rat eye for the first time. METHODS Retinal video images of 9 Wistar-Kyoto (WKY) rats were captured at a rate of 250 frames per second for 10s with a 50° field of view using a high speed camera (Optronis, Kehl, Germany). Two recordings were taken from each rat at the same sites for repeatability analysis. The electrocardiogram (ECG) was measured simultaneously with retinal images. Arterial retinal pulse wave velocity (rPWV) and arterial/venous pulse amplitude were calculated from recorded images. Arterial measurements were repeated in another normotensive strain of the same age (Sprague-Dawley, n=4). RESULTS The average WKY rPWV was 11.4 ± 6.1 cm/s. The differences between repeated measures were not significant (-2.8 ± 2.9 cm/s, p=0.2). Sprague-Dawley animals had a similar rPWV (9.8 ± 2.2 cm/s, p=0.61). The average arterial and venous pulse amplitude was 7.1 ± 1.5 μm and 8.2 ± 2.0 μm respectively. There was a positive correlation between rPWV and heart rate in the WKY groups (r(2)=0.32). A positive correlation was also obtained between arterial and venous diameter and their pulse amplitude (r(2)=0.67 and r(2)=0.37 respectively). CONCLUSION rPWV was associated with heart rate. Higher pulsation amplitude was also correlated with larger vessel diameter. High speed imaging of retinal vessels in the rat eye provides an accurate and robust method to study dynamic characteristics of these vessels and their relationship with ocular and systemic abnormalities.

A simplified method for quantifying the subject-specific relationship between blood pressure and carotid-femoral pulse wave velocity.

Devices that estimate blood pressure from arterial pulse wave velocity (PWV) potentially provide continuous, ambulatory blood pressure monitoring. Accurate blood pressure estimation requires reliable quantification of the relationship between blood pressure and PWV. Regression to population normal values or, when using limb artery PWV, changing hydrostatic blood pressure within the limb provides a calibration index. Population lookup tables require accurate anthropometric correlates, assuming no individual variation. Only devices that measure PWV in the limb can use limb position changes. This study proposes a method for developing a calibration curve independent of lookup tables and useful for large artery PWV measurement, such as carotid-femoral PWV (PWVcf). PWVcf was measured in 27 normal subjects (15 female, 36±19 years) in both the supine and standing position. The change in systemic pressure was measured and hydrostatic pressure change calculated from estimated vessel path length height, measured using body surface distances. Brachial diastolic blood pressure increased for all subjects from supine to standing (supine 70±8 mmHg, standing 83±8 mmHg, p<;0.001) with an additional hydrostatic change across the carotid-femoral path length of 19±2 mmHg (p<;0.001). PWVcf also increased in all subjects (supine 5.2±1.3 m/s, standing 7.3±2.2 m/s, p<;0.001). The subject-specific calibration index (ΔDP/ΔPWVcf) varied amongst the cohort (20±8 mmHg/m/s), was correlated with age (-0.57, p=0.002) and seated aortic systolic pressure (-0.38, p=0.048) and was always greater than zero. Thus, this study describes a simple but novel method of measuring an individualized calibration index using blood pressure and PWV measurements in the supine and standing position.

Field stimulation of the carotid baroreceptor complex does not compromise baroreceptor function in spontaneously hypertensive rats

Field stimulation of the carotid baroreceptors has been successfully used to induce a long-term reduction in blood pressure. However, baroreceptor stimulation may interfere with or compromise the beneficial short-term blood pressure regulation function of the baroreceptors. This study aims to quantify the baroreceptor function before and during acute, unilateral field stimulation of the carotid baroreceptors. Spontaneously Hypertensive Rats (n=7) were anaesthetised and instrumented to measure heart rate and mean arterial pressure (MAP), aortic pulse wave velocity (a surrogate measure of arterial stiffness), abdominal aortic flow and renal artery flow. A custom made field stimulation device was fitted to the left common carotid artery. Baroreceptor function was measured by quantifying heart rate response to MAP change induced by bolus injection of phenylephrine. Field stimulation of the baroreceptors reduced heart rate by 20 bpm (p=0.003) with MAP reduction of 18 mmHg (p=0.008). Maximal barorecep-tor gain without stimulation was -1.20±0.41 bpm/mmHg and during stimulation -1.41±0.52 bpm/mmHg (p=0.59). The MAP at which maximal gain occurred also did not change (152±11, 160±9 mmHg respectively, p=0.22). This study indicates that unilateral field stimulation of the carotid baroreceptor complex, while causing a sustained reduction of arterial pressure, does not alter acute baroreceptor function peak gain.

Increased arterial stiffness does not respond to renal denervation in an animal model of secondary hypertension

Renal denervation is a novel device based therapy promoted to reduce high blood pressure. We examined the impact of renal denervation on systolic blood pressure, renal function, and arterial stiffness in the Lewis Polycystic Kidney disease (LPK) rodent model of kidney disease. Animals were subjected to bilateral renal denervation or sham surgeries at age 6 and 12 weeks. Systolic blood pressure was monitored by tail-cuff plethysmography and renal function by urinalysis and creatinine clearance. At age 16 weeks, beat-to-beat aortic pulse wave velocity as a functional indicator of arterial stiffness was determined. Renal denervation produced an overall reduction in blood pressure in the LPK [(denervated 164±4 vs. sham-operated 180±6 mmHg, n = 6 per group, P=0.003)] and delayed, but did not prevent, the decline in renal function. Aortic pulse wave velocity was markedly elevated in the LPK compared with Lewis and was not altered by renal denervation in the LPK however a reduction was seen in the control Lewis animals. These results support the hypothesis that renal nerves contribute to secondary hypertension in conditions such as kidney disease.

Quantification of peripheral and central blood pressure variability using a time-frequency method

Systolic blood pressure variability (BPV) is associated with cardiovascular events. As the beat-to-beat variation of blood pressure is due to interaction of several cardiovascular control systems operating with different response times, assessment of BPV by spectral analysis using the continuous measurement of arterial pressure in the finger is used to differentiate the contribution of these systems in regulating blood pressure. However, as baroreceptors are centrally located, this study considered applying a continuous aortic pressure signal estimated noninvasively from finger pressure for assessment of systolic BPV by a time-frequency method using Short Time Fourier Transform (STFT). The average ratio of low frequency and high frequency power band (LFPB/HFPB) was computed by time-frequency decomposition of peripheral systolic pressure (pSBP) and derived central aortic systolic blood pressure (cSBP) in 30 healthy subjects (25-62 years) as a marker of balance between cardiovascular control systems contributing in low and high frequency blood pressure variability. The results showed that the BPV assessed from finger pressure (pBPV) overestimated the BPV values compared to that assessed from central aortic pressure (cBPV) for identical cardiac cycles (P<;0.001), with the overestimation being greater at higher power.Systolic blood pressure variability (BPV) is associated with cardiovascular events. As the beat-to-beat variation of blood pressure is due to interaction of several cardiovascular control systems operating with different response times, assessment of BPV by spectral analysis using the continuous measurement of arterial pressure in the finger is used to differentiate the contribution of these systems in regulating blood pressure. However, as baroreceptors are centrally located, this study considered applying a continuous aortic pressure signal estimated noninvasively from finger pressure for assessment of systolic BPV by a time-frequency method using Short Time Fourier Transform (STFT). The average ratio of low frequency and high frequency power band (LFPB/HFPB) was computed by time-frequency decomposition of peripheral systolic pressure (pSBP) and derived central aortic systolic blood pressure (cSBP) in 30 healthy subjects (25-62 years) as a marker of balance between cardiovascular control systems contributing in low and high frequency blood pressure variability. The results showed that the BPV assessed from finger pressure (pBPV) overestimated the BPV values compared to that assessed from central aortic pressure (cBPV) for identical cardiac cycles (P<;0.001), with the overestimation being greater at higher power.

Hemodynamics changes with acute carotid baroreceptor field stimulation are age-dependent in normotensive rats*

Carotid baroreceptor stimulation can treat resistant hypertension with possible effects on the vasculature beyond the decrease in arterial pressure. This study aims to characterize age-dependency of vascular hemodynamics changes with unilateral field stimulation of carotid baroreceptors in normotensive rats to infer underlying hemodynamic mechanisms. Anesthetized Wistar-Kyoto rats divided into two groups (young: n=10, 13-33 weeks; old: n=6, 52-58 weeks) were instrumented to measure heart rate (HR) and mean arterial pressure (MAP), and flow in the abdominal aorta and renal artery. Measures of aortic and renal artery stiffness and resistance were calculated. Baroreceptor stimulation caused a consistent reduction in MAP, HR, pulse pressure and aortic pulse wave velocity. In young rats reduced MAP (77±10 to 64±13 mmHg, p<;0.001) was concomitant with reduced mean aortic (40±15 to 32±11 ml/min, p<;0.05) and renal flow (3.0±1.6 2.2±1.1 ml/min, p<;0.001). However, in old rats reduced MAP (76±14 to 64±10 mmHg, p<;0.05) occurred with a reduced aortic resistance (1.8±0.9 to 1.6±0.9 mmHg.min/ml, p<;0.05), renal artery resistance (17.4±2.4 to 16.5±2.3 mmHg.min/ml, p<;0.05) and mean renal flow (4.5±1.2 to 4.0±1.1 ml/min, p<;0.05). This was consistent with reduced characteristic impedance in older rats in both the aorta (0.17±0.08 to 0.13±0.08 mmHg.min/ml, p<;0.05) and renal artery (4.97±1.16 to 3.97±1.08 mmHg.min/ml, p<;0.05). Stimulation caused a leftward shift in renal impedance phase frequency spectrum in both age groups indicating changes in wave reflection from the renal bed. Findings show that the reduction in MAP due to carotid barostimulation is associated with different hemodynamic mechanisms that depend on age.

Baroreceptor function is preserved following field stimulation of carotid baroreceptors in normotensive and hypertensive rats

### CONTRIBUTION OF THE AREA POSTREMA TO THE INCREASED CARDIAC SYMPATHETIC NERVE ACTIVITY IN OVINE HEART FAILURE Abukar Y, Ramchandra R, May CN The Florey Institute of Neuroscience and Mental Health, Melbourne, Victoria, Australia Background : Heart failure (HF) is associated with an increase in cardiac sympathetic nerve activity (CSNA), which is directly linked to mortality in HF patients. The mechanisms responsible for the elevated CSNA remain unclear. Previous studies indicate that the area postrema (AP), a circumventricular organ in the brainstem, plays a role in the control of sympathetic nerve activity. We hypothesized that the elevated CSNA in HF is mediated by the AP and lesioning this region would reduce the increased CSNA in sheep with HF. Aims : To determine the effect of sham lesion or lesion of the AP on CSNA and hemodynamics in conscious sheep with HF. Methods : Studies were conducted in 2 groups of sheep with pacing-induced HF: sham (n=6) and AP lesion (n=6) sheep. Mean arterial blood pressure (MAP), heart rate (HR) and CSNA were recorded simultaneously in conscious sheep at least 4 days after surgery. Results : Heart failure was associated with a significant decrease in ejection fraction (from 74±2 % to 38±1 %; P<0.001), which was similar in both groups. There was a significant reduction in CSNA burst incidence in the AP lesion group compared with the sham group (45±10 and 89±3 bursts/100 heartbeats, respectively; P<0.01). Conclusions : In sheep with HF, the group with lesion of the AP had a significantly lower CSNA compared with the sham group. These data suggest that the AP plays a role in setting the detrimental high levels of CSNA in HF. ### G PROTEIN-COUPLED ESTROGEN RECEPTOR SIGNALING IMPROVES STROKE OUTCOME IN FEMALE MICE Broughton BRS, Jansen GL, Sobey CG Department of Pharmacology, Monash University, Clayton, Victoria, Australia Background: Estrogen has been assumed to provide neuroprotection following stroke entirely via classical estrogen receptors. Interestingly, there is recent evidence that activation of a novel G …

Assessment of baroreflex sensitivity by continuous noninvasive monitoring of peripheral and central aortic pressure.

Noninvasive assessment of baroreceptor sensitivity (BRS) facilitates clinical investigation of autonomic function. The spontaneous sequence method estimates BRS using the continuous measurement of arterial pressure in the finger. Since the baroreceptors are centrally located (aortic arch, carotid arteries), this study assessed the use of a continuous aortic pressure signal derived from the peripheral pressure pulse to compute the BRS from changes in systolic pressure (SBP) and pulse interval (PI). BRS computed from central aortic (cBRS) and peripheral pressure (pBRS) was calculated in 12 healthy subjects (25-62 years, 7 females). The difference between pBRS and cBRS was calculated for four levels of pulse lags between changes in SBP and PI. For each lag and for the pooled data for all lags, cBRS was significantly correlated with pBRS (r2=0.82). The within subject difference ranged from -41.2% to 59.2%. This difference was not related to age, gender of hemodynamic parameters (systolic or diastolic pressure, heart rate, aortic pulse wave velocity). However 18.2% of the variance was due to the difference in the number of spontaneous pulse sequences used to determine values of cBRS and pBRS. The differences between pBRS and cBRS are in the range of values of BRS as those found, in other studies, to discriminate between patient groups with different levels of autonomic function. Findings of this study suggest that, given the heart rate dependent amplification of the arterial pressure pulse between the central aorta and the peripheral limbs, BRS determined from central aortic pressure derived from the peripheral pulse may provide an improved method for noninvasive assessment of baroreceptor function.